Revision as of 00:35, 2 December 2012

Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

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Swap space

Swap space will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support).

To check swap status, use:

$ swapon -s

Or：

$ free -m

Note: There is no performance advantage to either a contiguous swap file or a partition, both are treated the same way.

Swap partition

A swap partition can be created with most GNU/Linux partitioning tools (e.g. fdisk, cfdisk). Swap partitions are designated as type 82.

Note: If using a TRIM supported SSD, discard is a valid mount option for swap. If creating swap manually, using -d or --discard achieves the same. For more information and other available mount options, see the swapon man page.

Swap file

As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD).

Note: The BTRFS filesystem does not currently support swapfiles.

Swap file creation

As root use fallocate to create a swap file the size of your choosing (M = Megabytes, G = Gigabytes) (dd can also be used but will take longer). For example, creating a 512 MB swap file:

Note: Please note that in kernel resume parameter you still have to type path to partition (e.g. resume=/dev/sda1) not to swapfile explicitly! Parameter resume_offset is for informing system where swapfile starts on hard disk (e.g. resume_offset=7546880).

Swap with USB device

Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, USB device can be used as partition temporally. But this method has some severe disadvantage：

USB device is slower than hard disk.

flash memories have limited write cycles. Using it as swap partition will kill it quickly.

when another device is attached to the computer, no swap can be used.

To add a a USB device to SWAP, first take a USB flash and partition it with a swap partition.You can use graphical tools such as Gparted or console tools like fdisk. Make sure to label the partition as SWAP before writing the partition table.

Make sure you are writing the partition to the correct disk!

Next edit the fstab

# nano /etc/fstab

Now add a new entry, just under the current swap entry, which take the current swap partition over the new USB one

UUID=... none swap defaults,pri=10 0 0

where UUID is taken from the output of the command

ls -l /dev/disk/by-uuid/ | grep /dev/sdc1

Just replace sdc1 with your new USB swap partition. sdb1

We use UUID because when you attach other devices to the computer it could modify the device order

Last, add

pri=0

in the original swap entry for teaching fstab to use HD swap only when USB is full

This guide will work for other memory such as SD cards, etc.

Performance Tuning

Swap values can be adjusted to help performance.

Swappiness

The swappinesssysctl parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems.

/etc/sysctl.conf

vm.swappiness=1
vm.vfs_cache_pressure=50

Priority

If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk (/dev/sda) and a slower disk (/dev/sdb), assign a higher priority to the swap area located on the faster device. Priorities can be assigned in fstab via the pri parameter: